Collision response is different from Box2D

I work on a client/server game. The server uses the original Box2D implementation, and the client uses planck.js. I had some major differences in collision response between Box2D and planck. the test involves a (circle) bullet bouncing on a (circle) static body.

I identified the issue in the getWorldManifold function. I’ve only worked on circles for now, but the implementation in planck is different from the one in Box2D

    var pointA = Transform.mulVec2(xfA, this.localPoint);
    var pointB = Transform.mulVec2(xfB, this.points[0].localPoint);
    var dist = Vec2.sub(pointB, pointA);
    if (Vec2.lengthSquared(dist) > Math.EPSILON * Math.EPSILON) {
      normal.set(dist);
      normal.normalize();
    }
    points[0] = Vec2.mid(pointA, pointB);
    separations[0] = -radiusB - radiusA;

First Math.EPSILON does not exists and the right value should be Number.EPSILON (with a needed polyfill for Internet Explorer) I had not seen this so it’s probably valid, I was testing outside of planck.

The end of the code should also be something like this:

            var cA = pointA.clone().addMul(radiusA, normal);
            var cB = pointB.clone().addMul(-radiusB, normal);
            points[0] = Vec2.mid(cA, cB);
            separations[0] = Vec2.dot(Vec2.sub(cB, cA), normal);

Replacing the code got me the expected result (similar collision response as Box2D)

Is there any reason the implementation in planck looks so wrong? Could it be changed?

[Kal-Torak]

Oh, very nice, I’ll idle there then 👍

[shakiba]

Merged, thank you @FlorentMasson !